I. Allekotte

Underground muon counters as a tool for composition analyses

Abstract The transition energy from galactic to extragalactic cosmic ray sources is still uncertain, but it should be associated either with the region of the spectrum known as the second knee or with the ankle. The baseline design of the Pierre Auger Observatory was optimized for the highest energies. The surface array is fully efficient above 3xa0×xa010 18 xa0eV and, even if the hybrid mode can extend this range below 10 18 xa0eV, the second knee and a considerable portion of the wide ankle structure are left outside its operating range. Therefore, in order to encompass these spectral features and gain further insight into the cosmic ray composition variation along the transition region, enhancements to the surface and fluorescence components of the baseline design are being implemented that will lower the full efficiency regime of the Observatory down to ∼10 17 xa0eV. The surface enhancements consist of a graded infilled area of standard Auger water Cherenkov detectors deployed in two triangular grids of 433xa0m and 750xa0m of spacing. Each surface station inside this area will have an associated muon counter detector. The fluorescence enhancement, on the other hand, consists of three additional fluorescence telescopes with higher elevation angle (30°–58°) than the ones in operation at present. The aim of this paper is threefold. We study the effect of the segmentation of the muon counters and find an analytical expression to correct for the under counting due to muon pile-up. We also present a detailed method to reconstruct the muon lateral distribution function for the 750xa0m spacing array. Finally, we study the mass discrimination potential of a new parameter, the number of muons at 600xa0m from the shower axis, obtained by fitting the muon data with the above mentioned reconstruction method.

Enhancing the Pierre Auger Observatory to the 1017-1018.5 eV range : Capabilities of an Infill surface Array

Abstract The Pierre Auger Observatory has been designed to study the highest-energy cosmic rays in nature ( E ⩾ 10 18.5 xa0eV). The determination of their arrival direction, energy and composition is performed by the analysis of the atmospheric showers they produce. The Auger Surface Array will consist of 1600 water Cherenkov detectors placed in an equilateral triangular grid of 1.5xa0km spacing. The aim of this paper is to show that the addition of a “small” area of surface detectors at half or less the above mentioned spacing would allow a dramatic increase of the physical scope of this Observatory, reaching lower energies at which the transition from galactic to extragalactic sources is expected.

Acceptance and Angular Resolution of an Infill Array for the Pierre Auger Surface Detector

The Pierre Auger Observatory has been designed to study the highest‐energy cosmic rays in nature (E ⩾ 1019 eV). The determination of their arrival direction, energy and composition is performed by the analysis of the atmospheric showers they produce. The Auger Surface Array will consist of 1600 water Cerenkov detectors placed in an equilateral triangular grid of 1.5 km. In this paper we show how adding a “small” area of surface detectors at half the above mentioned spacing would make it possible to lower the detection threshold by one order of magnitude, thus allowing the Observatory to reach lower energies where the cross‐over from galactic to extragalactic sources is expected. We also analyze the angular resolution that can be attained with such an infill array.